1. Field of the Invention
This invention relates to method and apparatus for removing contaminants, such as NOx, SOx, particulates, heavy metals, and other acid gases from flue gas streams arising from industrial combustion processes and, more particularly, to an improved method for removing NOx from a flue gas stream by partial oxidation with ozone.
2. Description of the Prior Art
Nitrogen oxides sulfur oxides (SOx), particulates, heavy metals, and other acid gases are the main pollutants found in flue gases from chemical and combustion processes. The combustion and chemical processes generate flue streams with contaminants that need to be removed or cleaned-up before the flue gas is exhausted to the atmosphere. It is well known to remove nitrogen oxides from flue gas by a number of dry and wet processes, and sulfur oxides are removed by dry or wet scrubbing. Aqueous scrubbing is conventionally utilized to remove acid gases, such as SOx, Cl2, HCl, etc. particulates and other components. Nitric oxide, NO, is a major component of (NOx) in combustion processes, and because it is almost insoluble, removal by aqueous scrubbing is negligible. Further, limited success has been achieved in using reagents for scrubbing NOx.
Nitrogen oxides (NOx) are generally formed in flue gas streams arising from combustion processes due to a number of factors, such as high flame temperature, nitrogenous compounds present in the fuel, and nitrogenous content of material subjected to combustion temperature, such as encountered with the incineration of waste. Nitrogen oxides formed at temperatures above 1,300° F. are mainly in the form of NO. Sulfur compounds in fuel convert to form SOx. Other heteroatom compounds present in fossil fuel or combustion charge, such as chlorine, result in Cl2 or HCl. Combustion of coal, solid fuel, or charge to a kiln or furnace generates particulate matter and other contaminants, such as heavy metals (Hg) which may or may not be effectively removed by aqueous scrubbing.
Known absorption processes that remove NOx from gas streams by contacting the NOx with ozone as well known in the art are disclosed in U.S. Pat. Nos. 5,206,002; 6,162,409; and 7,303,735. These processes utilize a multi-pollutant removal approach that has been implemented in the removing NOx from flue gas arising from gas fired boilers and removing multiple pollutants, including NOx, SOx, particulates, etc. in coal fired boilers, metal pickling processes, fluidized catalytic crackers, regenerators, heavy metal furnaces, and the like.
With the processes disclosed in the above patents, NOx is reacted with ozone forming higher order oxides of nitrogen, specifically, pentavalent form (N2O5) or higher which are very soluble and are easily removed by wet scrubbing. In these processes, the stoichiometeric amount of ozone required to convert one mole of NOx to pentavalent form is about 1.5 moles of ozone. Although the known methods are very effective in achieving ultra low levels of NOx emissions in the treated gas stream, the cost of ozone makes the processes prohibitively expensive, especially when the gas streams have high levels of NOx, to begin with and the processes generate nitrate/nitric acid in the scrubber purge, requiring disposal in an environmentally safe manner or that they be utilized in the fabrication of a by-product.
Other known processes for the oxidation of NOx to NO2 by the addition of ozone are disclosed in U.S. Pat. Nos. 4,011,298; 4,035,470; 4,107,271; 4,119,702; 4,247,321; 4,541,999; and 4,564,510. With these processes, oxidized NOx is absorbed or reacted with various reagents. The patents teach ozone oxidation of NOx. The removal of NOx increases with an increase in the amount of ozone added. The processes rely upon reaching higher oxides of NOx to effectively scrub the NOx from the flue gas stream. The scrubber purge produced in these processes is a mixture of various salts in either aqueous solution or slurry containing sulphite, sulphate, nitrite, nitrate, chlorides, or acids, which are difficult to treat and manage in a waste water treatment plant. With the prior art methods at molar ratios of approximately 0.5 removal efficiencies are very low and are not particularly successful in attaining the required NOx removal without creating a significant amount of secondary purge streams.
NOx in a partially oxidized form (trivalent and tetravalent form) has a lower solubility than pentavalent form and scrubbing is less effective, especially when the concentration of NOx is low. Using alkali or alkaline earth metal carbonates, bicarbonates or hydroxide as scrubbing reagents improves removal efficiencies. When partially oxidized NOx is absorbed in alkaline solution both nitrate and nitrite are formed in various concentrations. Suchak et al. discloses in “Absorption Nitrogen Oxides in Alkaline Solutions Selective Manufacture of Sodium Nitrite”, Ind.Eng.Chem.Res., vol.29, pgs. 1492-1502 (1990) the method and parametric conditions for selectively making sodium nitrite using partially oxidized NOx containing process gas. Nitrite formation can be enhanced by preferential formation and transport of nitrous acid (HNO2) in the gas phase into an alkaline medium to form nitrite.
In the absence of an alkali/alkaline, carbonate/hydroxide, nitrous acid in an aqueous medium is unstable in both neutral and acidic pH. ‘Nitrous acid breaks down or decomposes into nitric acid (HNO3) and nitric oxide (NO). Nitric oxide is sparingly soluble and, therefore, is released back to the gas phase while nitric acid remains in the solution.
Therefore, there is need for an improved process for removing contaminants, that includes higher concentrations of NOx, with ozone in a cost effective manner that substantially minimizes or eliminates the formation of nitrate in the purge stream from a wet scrubber.